1. Field of the Invention
The present invention relates in general to stepping motors for use in controlling the position of a driven object and, more particularly, to improvement of a stepping motor that enables the position of a vehicle lamp unit to be suitably controlled with a simple mechanism.
2. Description of the Related Art
FIG. 9 shows a stepping motor according to the related art disclosed in Japanese Laid-Open Patent Application No.9-149621. Referring to FIG. 9, the stepping motor comprises a housing 32, an inner space 33 formed inside the housing 32, a magnetic field generating coil 34 formed along the entirety of the periphery of the inner space, a rotor 35 formed as a piece of permanent magnet facing the magnetic field generating coil 34 and a bearing member 36 for rotatably securing the rotor 35 to the housing 32.
A description will now be given of the operation of the stepping motor according to the related art.
A current is supplied to the magnetic field generating coil 34 so as to generate a magnetic field. The rotor 35 is rotated as a result of repulsion and attraction occurring between the magnetic field and the permanent magnet constituting the rotor 35. When the magnetic field generated by the field magnetic generating coil 34 and that of the permanent magnet interact to produce a magnetically stable condition, the rotor 35 stops rotating.
By allowing a driven object (not shown) to follow the rotation of the rotor 35, the driven object can be controlled to attain positions corresponding to respective angles that the rotor 35 forms in a magnetically stable condition.
Since the stepping motor according to the related art is constructed as described above, a retaining force sufficient to hold the rotor 35 stationary with respect to the housing 32 is not available when the supply of the current is terminated.
There are many factors that cause an object driven by a stepping motor to vary its position. For example, a vehicle lamp unit is subject to such a variation due to a vibration load from the vehicle movement and due to reduction in the magnetic force occurring as a result of an increase in the ambient temperature. Therefore, the vehicle lamp unit cannot be held in a target position without continuously supplying the current to the stepping motor.
Another aspect relating to the related art stepping motor is that it is usually subject to an open loop control. When an open loop control is used to control the position of the driven object, an initializing process should be undertaken in which the motor is rotated in a predetermined direction until the rotor 35 and follower members (i.e. objects driven by the rotor 35) come into contact with a stopper. When such an initial position setting is undertaken, variations are produced in the magnitude of repulsion after the contact. Such variations are caused by differences in rotating speed of the motor when the contact occurs, variations in the hardness of the contacting members, or deformation of the contacting members. Consequently, variations exist in the initial positions of the rotor 35 and those of the driven object after the initial position setting.
Variations in the initial positions and absence of a sufficient retaining force are disadvantageous in that, for example, the vehicle lamp unit cannot be held in a desired position in accordance with the position of the vehicle. This means that, if the lamp unit includes a High Intensity Discharge (HID) lamp, it cannot be properly controlled to prevent a driver on an opposite lane from being dazzled.
FIG. 10 is a graph showing a damping characteristic of a stepping motor according to the related art, in which time is plotted horizontally and positions are plotted vertically. The solid line represents a characteristic for position control and the dotted line represents a characteristic for initial position setting. A maximum amplitude difference .DELTA..theta.1 is produced between an overshoot and an undershoot when the rotor is rotated one step. A maximum undershoot .phi.1 is produced as a result of repulsion after the initial position setting. The magnitude of the maximum undershoot .phi.1 affects the variations in the initial positions of the rotor 35 after the initial position setting.
Japanese Laid-Open Patent Application No. 9-149621 discloses a damper construction in which a plate spring 37 is provided adjacent to the rotor 35, on one side of the housing 32 and a fluorine resin annular plate washer 38 is provided on the other side. The rotor 35 is pressed toward the housing 32 via the fluorine resin annular washer 38. It is expected in Japanese Laid-Open Patent Application NO. 9-149621 that the above-described disadvantage is eliminated using the damping capability and the retaining force provided by such a damping construction.
When the lamp unit using an HID lamp is controlled so as to prevent a driver in an opposite lane from being dazzled, the magnitude of resistance to movement, produced by sliding contact, should be properly controlled. Merely disposing an annular washer 38 between the rotor 35 and the housing 32 would not serve the purpose. In Japanese Laid-Open Patent Application No. 9-149621, the annular washer 38 and the rotor 35 are in face contact with each other. The annular washer 38 and the housing 32 are also in face contact with each other. Therefore, the resistance to movement can only be controlled by appropriately selecting a material forming the annular washer 38 or by selecting a substance used in coating the annular washer 38. Work required to control the magnitude of the resistance is extensive and time-consuming. Moreover, selection of an appropriate material that serves the purpose may sometimes be impossible.